Higgs particle
This short chapter tries to give you some insight into the search for this new particle, which has been recently discussed quite a lot.


While LHC is searching for many new particles the Higgs particle is the most famous candidate. It's discovery would proof a theory that was put up in the mid-1960's describing a mechanism that gives mass to all particles of the standard model. One can derive from symmetries that all elementary particles were massless immediately after the big bang. The theory suspects that a viscous medium called the Higgs field spread out through our universe a trillionth of a second after the big bang. Since that time particles have masses different from zero: The more they interact with this medium, i.e the larger their masses are, the more force is needed in order to accelerate them inside this medium. To proof the higgs field it has to be excited in some way, like creating little curls in a fluid or gas. These curls are the quanta of the Higgs field excitations and called Higgs particles. They are massive and extremely short-lived. Before they can reach the detectors they decay into other particles so that they can only be found on the basis of their decay products. Into which other particles a Higgs particle decays, predictably depends on its mass. But this value is as yet unknown. That is why physicists are looking for various signals that belong to all possible decay modes of the Higgs particle. In the diagram below that is based on theoretical calculations, the fractions (y axis) of the most important decay processes of the Higgs particle are shown. The influence of the Higgs mass (x axis) on these fractions become clear as well.



Pay attention to the blue, dashed line (indicated with WW). It tells us: If the Higgs mass is at least 140GeV/c2, it will decay mostly into two W particles. Since the Higgs particle is electric neutral these W's have opposite electric chargees.

You now know what you need to track down events that result from a Higgs decay. Since you know the signal of a W particle when it was produced in the collision you will be able to pick out events with two W particles. Here are two more Feynman diagrams showing the production and decay of the Higgs particle and one background event (in this case: production of a heavy top quark pair).





And now, you can find this particle – let's go to the measurements!